This invention relates to a control valve for the braking system of a tractor-trailer combination vehicle having air brakes.
Existing Federal braking regulations require that tractor-trailer combination vehicles be equipped with air brakes and that both the tractor and the trailer be equipped with spring-applied, fluid pressure released parking brakes, commonly known as "spring brakes". In other words, the spring brakes are released when the vehicle is to be moved by communicating hold-off pressure to the brakes to thereby overcome the force of the spring, and the spring brakes are applied when the vehicle is parked by venting this hold-off pressure, thereby permitting the springs to apply the brakes. Federal regulations further require a single control valve capable of applying the spring brakes on both the tractor and trailer simultaneously and a second control valve capable of controlling the spring brakes on the trailer only. In addition to these two required control valves, it has become customary to provide a third valve that permits the spring brakes on the tractor to be applied while the spring brakes on the trailer are released, to thereby permit charging of the trailer braking system while the tractor spring brakes are applied, and also to permit use of air actuated accessories (such as rear lift gates) while the vehicle is parked with the tractor parking brakes. However, the addition of the third valve, of course, requires additional fluid connections with potential leakage points and also substantially increases the cost of the system. Furthermore, the aforementioned third valve in many such systems may be inadvertently operated while the vehicle is in motion, thereby creating a very dangerous condition with the tractor brakes applied and the trailer brakes released while the vehicle is in motion.